Laminated package

ABSTRACT

A system and method of aligning a micromirror array to the micromirror package and the micromirror package to a display system. One embodiment provides a method of forming and utilizing a package that exposes regions of an alignment reference plane. The device within the package is mounted on the reference plane such that the exposed regions allow precise alignment with the device in a direction perpendicular to the reference plane. Alignment surfaces formed in a display system or other system contact the reference plane at the exposed regions to position the packaged device relative to other components of the system. One embodiment of the package  400  taught has laminated layers forming the package substrate  402  and providing a precision reference plane  416  relative to the position of the micromirror device  404.  The package may be formed by laminating several layers of material in sheets to form several package substrates simultaneously. Voids formed in the layers  408  on one side of the reference plane provide access to the reference plane. A transparent cover or lid  412  is attached to the package substrate  402  sealing the micromirror  404  in the cavity  410.  The preceding abstract is submitted with the understanding that it only will be used to assist in determining, from a cursory inspection, the nature and gist of the technical disclosure as described in 37 C.F.R. §1.72(b). In no case should this abstract be used for interpreting the scope of any patent claims.

This application claims priority under 35 USC § 119(e)(1) of provisionalapplication No. 60/258,994 filed Dec. 29, 2000.

FIELD OF THE INVENTION

This invention relates to the field of packaging optical systems, moreparticularly to alignment of small optical components such asmicromirrors, detectors, and reflectors within their packages.

BACKGROUND OF THE INVENTION

Micromechanical devices are small structures typically fabricated on asemiconductor wafer using techniques such as optical lithography,doping, metal sputtering, oxide deposition, and plasma etching whichhave been developed for the fabrication of integrated circuits.

Micromirrors are a type of micromechanical device. Other types ofmicromechanical devices include accelerometers, pressure and flowsensors, gears and motors. While some micromechanical devices, such aspressure sensors, flow sensors, and micromirrors have found commercialsuccess, other types have not yet been commercially viable.

Micromirrors are primarily used in optical display systems. In displaysystems, the micromirror is a light modulator that uses digital imagedata to modulate a beam of light by selectively reflecting portions ofthe beam of light to a display screen. While analog modes of operationare possible, commercially feasible micromirrors typically operate in adigital bistable mode of operation and as such are the core of the firsttrue digital full-color image projection systems.

Micromirrors have evolved rapidly over the past ten to fifteen years.Early devices used a deformable reflective membrane which, whenelectrostatically attracted to an underlying address electrode, dimpledtoward the address electrode. Schlieren optics illuminate the membraneand create an image from the light scattered by the dimpled portions ofthe membrane. Schlieren systems enabled the membrane devices to formimages, but the images formed were very dim and had low contrast ratios,making them unsuitable for most image display applications.

Later micromirror devices used flaps or diving board-shaped cantileverbeams of silicon or aluminum, coupled with dark-field optics to createimages having improved contrast ratios. Flap and cantilever beam devicestypically used a single metal layer to form the top reflective layer ofthe device. This single metal layer tended to deform over a largeregion, however, which scattered light impinging on the deformedportion. Torsion beam devices use a thin metal layer to form a torsionbeam, which is referred to as a hinge, and a thicker metal layer to forma rigid member, or beam, typically having a mirror-like surface:concentrating the deformation on a relatively small portion of themicromirror surface. The rigid mirror remains flat while the hingesdeform, minimizing the amount of light scattered by the device andimproving the contrast ratio of the device.

Recent micromirror configurations, called hidden-hinge designs, furtherimprove the image contrast ratio by fabricating the mirror on a pedestalabove the torsion beams. The elevated mirror covers the torsion beams,torsion beam supports, and a rigid yoke connecting the torsion beams andmirror support, further improving the contrast ratio of images producedby the device.

Micromirror arrays used in display systems are small compared to mostother technologies. The small array size makes the alignment of thearray critical to the performance of the device. A misalignment of only100 μm shifts the image more than eight image pixels. While thismisalignment requires larger, more expensive optics in sequential colorsystems that use a single micromirror, misalignment can complicate theconvergence operation in the larger three-micromirror display systems.The micromirror package are used to align the micromirror array to thedisplay system optics. Not only must the micromirror be preciselyaligned relative to the package, the plane of the micromirror must bealigned with the plane of the projection lens to achieve proper focusacross the entire micromirror array. What is needed is a method andsystem of ensuring precise alignment of the micromirror array within amicromirror package.

SUMMARY OF THE INVENTION

Objects and advantages will be obvious, and will in part appearhereinafter and will be accomplished by the present invention whichprovides a method and system for precision micromirror positioning. Oneembodiment of the claimed invention provides a method of forming andutilizing a package that exposes discrete regions of an alignmentreference plane. The device within the package is mounted on thereference plane such that the exposed regions allow precise alignmentwith the device in a direction perpendicular to the reference plane.Alignment surfaces formed in a display system or other system contactthe reference plane at the exposed regions to position the packageddevice relative to other components of the system. For example, a mountmay be positioned between a lens and a micromirror device packaged insuch a package. The exposed regions of the reference plane would enablesimple and accurate alignment of the projection lens with themicromirror allowing the projection lens to obtain a consistent focusacross the surface of the micromirror device. Often the mount issandwiched between the micromirror device and an intervening prismassembly.

Another embodiment of the present invention provides a method of forminga package substrate. The method comprises: providing sheets of substratelayers; forming metalized patterns on at least one of the sheets;laminating the sheets to form the package substrate, the sheets shapedto provide a substrate having a cavity, the cavity having a floordefining a reference plane, wherein the layers are shaped to exposeregions of the reference plane outside the cavity.

Another embodiment of the present invention provides a method ofpackaging a semiconductor device. The method comprising: providing apackage substrate having a cavity, the cavity having a floor defining areference plane, discrete surface regions of the reference plane exposedoutside the cavity; attaching a semiconductor device in the cavity ofthe package substrate; attaching a lid to the package substrate toenclose the device in the cavity.

Another embodiment of the present invention provides a method ofaligning a display system. The method comprising: providing positioningstructures defining a display system reference plane; aligning amicromirror package with the display system reference plane, themicromirror package having a reference plane defined by regions on acommon plane, a micromirror attached to at least one of the regions, thealignment of the micromirror package accomplished by placing at leasttwo of the regions defining the reference plane against the positioningstructures; positioning illumination optics relative to the displaysystem reference plane; and positioning projection optics relative tothe display system reference plane.

Another embodiment of the present invention provides a packagesubstrate, comprising: a bottom surface; a top surface opposing thebottom surface; a cavity open to the top surface, the cavity having afloor defining a reference plane; and discrete regions outside thecavity exposing a surface of the reference plane.

Another embodiment of the present invention provides a semiconductordevice, comprising: a package substrate, the package substrate having acavity, the cavity having a floor defining a reference plane, discreteregions of the reference plane exposed outside the cavity; asemiconductor device in the cavity of the package substrate; a lidattached to the package substrate enclosing the device in the cavity.

Another embodiment of the present invention provides a display system,comprising: positioning structures defining a display system referenceplane; a spatial light modulator package having a reference planedefined by discrete regions on a common plane, a spatial light modulatorattached to at least one of the regions, at least two of the regions ofthe spatial light modulator package against the positioning structures;illumination optics positioned relative to the display system referenceplane; and projection optics positioned relative to the display systemreference plane.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a side view of a projection lens with a cross section sideview of a packaged micromirror of the prior art.

FIG. 2 is a plan view of prior art package layers prior to assembly,after assembly, and after singulation.

FIG. 3 is a top view of the package layers forming the new package priorto assembly, after assembly, and after singulation.

FIG. 4 is a cross section side view of a micromirror package showing thelaminated layers forming the package substrate and providing a precisionreference plane relative to the position of the micromirror device.

FIG. 5 is a cross section side view of a micromirror package showing thelaminated layers forming the package substrate and providing a precisionreference plane relative to the position of the micromirror device.

FIG. 6 is a top view of the micromirror package of FIG. 4 showing thelocation of the reference plane regions around the package of FIG. 4.

FIG. 7 is a perspective view of the micromirror package of FIG. 6.

FIG. 8 is a side view of the micromirror package showing the alignmentof the micromirror package with the display optics using the regionsproviding access to the reference plane.

FIG. 9 is a schematic view of a display system using the improvedpackage according to the present invention.

FIG. 10 is a flow chart showing a method of forming a package substrate.

FIG. 11 is a flow chart showing a method of packaging a semiconductordevice.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A method and system for precision micromirror alignment has beendeveloped. The new method and system provide improved alignment of themicromirror device to display system optics. The improved alignment isachieved by exposing portions of a reference layer of the packagesubstrate. When the micromirror is packaged, it is placed on thereference layer. The reference layer is used to position the micromirrorrelative to the display optics, especially the projection lens. Thesurface of the same reference layer both supports the micromirror andpositions the package relative to the optics. This removes the tolerancebuild up accumulated by the intervention of package layers formed on topof, or beneath, the reference layer. The removal of this tolerancebuild-up provides improved control over the back focal length of theprojection lens and improves the image focus over the entire surface ofthe micromirror array.

FIG. 1 is a side view of a projection lens 102 with a cross section sideview of a packaged micromirror 104 of the prior art. In FIG. 1, themicromirror 104 is held in a cavity formed in a substrate 106, andcovered with a transparent lid 108. The micromirror must be held in aplane parallel to the principal plane of the lens, whether a single lensor a lens system, to achieve optimum focus of the array onto an imageplane.

Prior art display systems accomplished alignment of the micromirrorarray 104 relative to the principal plane of the lens 102 in one of twoways. Either the back surface 110, or bottom surface, of the micromirrorpackage was used as a reference, or the front surface 112, or topsurface, was used as to align the device. The surface used was pressedagainst another component in the display system chassis, and the rest ofthe optical system was aligned to this other component. For example,many of the micromirror display systems utilize a prism system betweenthe micromirror package and the projection lens. The micromirror packagewas pressed against the surface of the prism adjacent to themicromirror, or an intervening spacer, to align the micromirror packagewith the display system optics.

Other prior art method of improving the alignment of the micromirror tothe display system optics eliminates at least one source of variance inthe alignment of the display system by recessing the entire perimeter ofthe package. This recess exposes the layer of the package to which themicromirror is attached. Forming the recess allows the system to alignto the same layer of the package on which the micromirror ismounted-eliminating the variance in thickness of the upper portion ofthe package.

FIG. 2 is a plan view of prior art package layers prior to assembly,after assembly, and after singulation. In FIG. 2, the lower packagelayers are formed in an unfired ceramic sheet 202. The lower packagelayer sheet 202 typically includes several package layers sandwichingelectrical conductors. Top portions 204 of the package are formed andaligned on the lower package layer sheet 202. This assembly is thenfired and the individual package substrates 106 are separated from thefired sheet. While this package design improves the alignment of thedevice, it is expensive to manufacture in large part due to the handlingof the top portions 204 of the packages.

FIG. 3 is a plan view showing the layers of the new package prior toassembly, after assembly, and after singulation. In FIG. 3, the lowerpackage layers are formed in an unfired ceramic sheet 302. The lowerpackage layer sheet 302 typically includes several package layerssandwiching electrical conductors. A single upper package layer sheet304 is formed. The upper package layer sheet 304 includes voids 306 thatwill form the device cavity of the package, and voids 308 that willprovide access to the reference plane of the package. The upper packagelayer sheet 304 is aligned on the lower package layer sheet 302. Thisassembly is then fired and the individual package substrates 310 areseparated from the fired sheet. The package substrate 310 of the presentinvention, by providing limited recesses 312 to the reference planerather than access around the entire perimeter of the substrate is mucheasier and cheaper to manufacture.

FIG. 4 is a cross section side view of a new micromirror package 400showing the laminated layers forming the package substrate 402 andproviding limited access to a precision reference plane relative to theposition of the micromirror device 404. In FIG. 4, the package is formedby laminating seven layers of material. The number of package layers isarbitrary. Packages may have as few as two layers, or as many as arenecessary or desirable to form the package. In FIG. 4, the lower fourpackage layers 406 typically include metalization patterns used tointerconnect the micromirror array with external circuitry. The layerstypically are ceramic and are fired to produce a finished packagesubstrate. Alternatively, the package layers may be plastic. The upperthree package layers 408 form a cavity 410 in which the micromirror 404is held. A transparent cover or lid 412 is attached to the packagesubstrate 402 sealing the micromirror 404 in the cavity 410.

In FIG. 4, the upper three package layers 408 are removed from regionsof the package. These regions may be on the perimeter of the device, butdo not extend all around the perimeter of the device. The removal ofthese layers provides access to the upper surface 414 of the lowerlayers. The upper surface 414 of the lower layers is the reference plane416 to which the micromirror 404 is attached. In FIG. 4, the referenceplane is the interface between the upper layers 408 and the lower layers406. When installed in a display system, or other optical system, theexposed portions of the reference plane are used to align themicromirror package.

The package of FIG. 4 provides accurate alignment since the alignmentfeatures of the display system have direct access to the reference planeof the micromirror package 400. This eliminates any variance in thethickness of the upper layers 408 or in the transparent cover 412 indisplay systems that align to the transparent cover, or any variance inthe lower layers 406 in display systems that align to the bottom surfaceof the package. In FIG. 4, the only remaining variance between theactual micromirror device and the remainder of the display system is thethickness of the adhesive used to attach the micromirror to the packageand any cupping or lack of planarity in the upper surface of the lowerlayers.

FIG. 5 is a cross section side view of a micromirror package 500 showingthe laminated layers forming the package substrate and providing aprecision reference plane relative to the position of the micromirrordevice 404. In FIG. 5, portions of the lower package layers 506 havebeen removed to expose discrete regions of the lower surface 514 of theupper layers 508. The lower surface 514 of the upper layers is thereference plane. The reference plane again is the interface between theupper layers 508 and the lower layers 506 forming the package substrate.When installed in a display system, or other optical system, the exposedportions of the reference plane are used to align the micromirrorpackage.

The package of FIG. 5 provides a more accurate alignment since thealignment features of the display system have direct access to limitedregions of the reference plane 516 of the micromirror package 500. Thiseliminates any variance in the thickness of the upper layers 508 or inthe transparent cover 512 in display systems that align to thetransparent cover, or any variance in the lower layers 506 in displaysystems that align to the bottom surface of the package. In FIG. 5, theonly remaining variance between the actual micromirror device and theremainder of the display system is the thickness of the adhesive used toattach the micromirror to the package and any cupping or lack ofplanarity in the upper surface of the lower layers.

FIG. 6 is a top view of the micromirror package 602 similar to thatshown in FIG. 4 showing the location of the reference plane regionsaround the package of FIG. 4. FIG. 7 is a perspective view of themicromirror package 602. In FIGS. 6 and 7, the package is comprised of asubstrate 606 holding a micromirror device 608 in a cavity 610 coveredby a transparent cover 612.

Several regions 604 are defined around the perimeter of the package 602.These regions 604 provide limited access to the reference plane on whichthe micromirror is mounted. Typically three regions 604 are defined andused to position the micromirror package, but different numbers ofregions can be used. Because the access regions 604 do not extend aroundthe perimeter of the device, manufacture of the present substrate ismuch less expensive. FIG. 7 is a perspective view of the package of FIG.6.

These regions 604 are used not only to align the micromirror package toa display system, but also as a reference when mounting the micromirrorarray in the package. The automated mounting machinery accesses the sameregions in the same manner they will be accessed by the display systemto ensure a consistent placement of the micromirror array in thepackage. The use of these reference points ensures the micromirror arrayis parallel to, and a consistent distance from (due to the thickness ofthe adhesive), the reference plane.

FIG. 8 is a side view of the micromirror package showing the alignmentof the micromirror package 802 with the display optics using the regionsproviding access to the reference plane. In FIG. 8, a mount 804 spacesthe micromirror package 802 apart from a prism assembly 806. The mount804 contacts the exposed regions of the reference plane and the prismassembly to provide precise spacing between the micromirror array heldby the package and the prism assembly. The spacing between themicromirror array and the prism assembly is consistent across thesurface of the micromirror array. The display system chassis hold theprojection lens 808 in alignment with the prism assembly 806 allowingthe projection lens 808 to form a well focused image of the micromirrorarray on the image plane 810.

FIG. 9 is a schematic view of a display system using the improvedpackage according to the present invention. In FIG. 9, light from lightsource 904 is focused on the improved micromirror 902 by lens 906.Although shown as a single lens, lens 906 is typically a group of lensesand mirrors which together focus and direct light from the light source904 onto the surface of the micromirror device 902. As shown in FIG. 9,a color wheel 916, or another color splitting mechanism such as a colorsplitting prism is used to separate a beam of white light into separateprimary colored light beams.

Image data and control signals from controller 914 cause some mirrors inthe array to rotate to an on position and others to rotate to an offposition. Mirrors on the micromirror device that are rotated to an offposition reflect light to a light trap 908 while mirrors rotated to anon position reflect light to projection lens 910, which is shown as asingle lens for simplicity. Projection lens 910 focuses the lightmodulated by the micromirror device 902 onto an image plane or screen912.

FIG. 10 is a flow chart showing a method of forming a package substrate.In block 1002, substrate layers are provided. The layers typically arean unfired ceramic composition, often having metal layers on one or bothsurfaces. In block 1004, metalized patterns are formed on the substratelayers. The metalized patterns typically are formed by removing the bulkof the metalization so that only the regions forming the electricalinterconnects remain. The layers are then laminated, or stacked on topof each other, in block 1006 and cured to produce a finished packagesubstrate. The curing process typically involves firing the laid-upceramic layers.

At some point, typically prior to curing the package substrate, thelayers are shaped to create voids that will expose the reference planewhen the package substrate is completed. The voids may be formed whenthe layers are first created, or may be formed later in the process. Thevoids occur in the layers on one side of the reference plane to provideaccess to the reference plane from that side—either the top or thebottom of the package. Likewise, some of the layers typically alsocontain a void to form the package cavity into which the device beingpackaged will be placed.

FIG. 11 is a flow chart showing a method of packaging a semiconductordevice. A package substrate is provided in block 1102. The packagesubstrate has a cavity with a floor. The floor is a flat surface thatdefines a reference plane for the package. Outside of the cavity portionof the package substrate, regions of the reference plane areexposed—that is, regions of the package outside the cavity are formedwith surfaces coplanar to the floor of the cavity. The regions may beexposed to either the top or bottom side of the package substrate.Alternatively, some regions may be exposed to the top side while othersare exposed to the bottom side of the package substrate.

A semiconductor device is attached to the package substrate cavity inblock 1104. The semiconductor device often is some type ofelectro-optical device that requires precise alignment with objectsoutside the package. Typically the device is a micromirror device thatrequires precision alignment between the micromirror device an opticalcomponents outside the micromirror package.

After the semiconductor device is attached to the cavity, a lid isattached to enclose the device in the package cavity as shown in block1106. The lid typically is a piece of glass. The lid may be glued to thepackage substrate—which provides a semi-hermetic seal—or mounted in akovar or other frame and brazed to the remainder of the package.

The package described above improves the alignment of various displaysystem components to the micromirror device packaged in the improvedpackage. In display systems in which the micromirror array is imagedonto an image plane, it is critical that the array and the principalplane of the projection lens are parallel so that the micromirror arraycan be properly focussed across the entire face of the array.

Alignment within the display system typically is provided by matingoptical components to each other, or to fixed features molded into thedisplay chassis. The location of these features is determined by theassumed position of the micromirror array. Prior art systems used thetop or bottom of the micromirror package and assumed a known offset fromthe face of the array. Unfortunately, each layer of the package adds tothe variability of the package. Thus, if there are two ceramic layersforming the walls of the package cavity, each of the layers addsvariability to the distance between the face of the package and the faceof the spatial light modulator.

The disclosed package eliminates this source of variability by providingaccess to the same plane on which the micromirror is attached. Thisaccess allows other components, such as the mount of FIG. 8 or featuresof the system chassis, to make mechanical contact with the referenceplane, typically via protrusions engaging the reference regions.

A display system is aligned using the disclosed invention by defining adisplay system reference plane to which all of the illumination optics,projection optics, and the modulator will be aligned. Positioningstructures are provided to contact regions of the modulator referenceplane that are exposed. Placing the modulator against these positioningstructures aligns the modulator to the structures. The illuminationoptics and projection optics are then positioned relative to thepositioning structures. The optics are positioned either directly, as isthe case when a mount having the positioning structures is held betweenthe modulator and a prism, or indirectly when a series of alignmentfeatures are formed in a display system chassis and each of the featuresaligns a particular item.

Thus, although there has been disclosed to this point a particularembodiment for precision micromirror positioning and method therefore,it is not intended that such specific references be considered aslimitations upon the scope of this invention except insofar as set forthin the following claims. Furthermore, having described the invention inconnection with certain specific embodiments thereof, it is to beunderstood that further modifications may now suggest themselves tothose skilled in the art, it is intended to cover all such modificationsas fall within the scope of the appended claims. In the followingclaims, only elements denoted by the words “means for” are intended tobe interpreted as means plus function claims under 35 U.S.C. §112,paragraph six.

1. A method of forming a package substrate, the method comprising:providing sheets of substrate layers; forming metalized patterns on atleast one of said sheets; laminating said sheets to form said packagesubstrate, said sheets shaped to provide a substrate having a cavity,said cavity defining a reference plane, wherein said layers are shapedto expose limited regions of said reference plane outside said cavity;and separating said laminated sheets to form individual packagesubstrates.
 2. The method of claim 1, wherein said laminating saidsheets comprises laminating said sheets to form said package substrate,said sheets shaped to provide a substrate having a cavity surrounded bya substrate wall, said cavity defining a reference plane, wherein saidsheets are shaped to expose regions of said reference plane outside saidsubstrate wall.
 3. The method of claim 1, wherein said laminating saidsheets comprises laminating said sheets to form said package substrate,said sheets shaped to expose regions of said reference plane to asurface of said package substrate parallel to said reference plane. 4.The method of claim 1, said forming metalized patterns on at least oneof said sheets comprising forming metalized patterns on at least one ofsaid sheets to provide electrical connection between said cavity and anexternal surface of said package substrate.
 5. The method of claim 1,said providing substrate sheets comprising: providing ceramic substratesheets.
 6. The method of claim 1, said providing substrate sheetscomprising: providing plastic substrate sheets.
 7. A package substrate,comprising: a first surface; a second surface opposing said firstsurface; a cavity open to said second surface, said cavity defining areference plane; and regions outside said cavity exposing a discreteportions of a surface of said reference plane.
 8. The package substrateof claim 7, wherein said regions outside said cavity expose a surface ofsaid reference plane from a side corresponding to said second surface.9. The package substrate of claim 7, wherein said regions outside saidcavity expose a surface of said reference plane from a sidecorresponding to said first surface.
 10. The package substrate of claim7, said package substrate formed of a laminated series of layers, saidregions farmed by voids in said layers on one side of said referenceplane.
 11. The package substrate of claim 7, said package substrateformed of a laminated series of layers, said cavity and said regionsformed by voids in said layers on a side of said reference planecorresponding to said second surface.
 12. The package substrate of claim7, said package substrate formed of a laminated series of layers, saidcavity and said regions formed by voids in said layers on a side of saidreference plane corresponding to said first surface.